Throughout this application various patents and published patent applications are referred to by an identifying citation. The disclosures of the patents and published patent applications referred to in this application are hereby incorporated into the present disclosure by reference to more fully describe the state of the art to which this invention pertains.
An electroactive material that has been fabricated into a structure for use in a battery is referred to as an electrode. Of a pair of electrodes used in a battery, herein referred to as a chemical source of electrical energy, the electrode on the side having a higher electrochemical potential is referred to as the positive electrode, or the cathode, while the electrode on the side having a lower electrochemical potential is referred to as the negative electrode, or the anode.
An electrochemically active material used in the cathode or positive electrode is referred to hereinafter as a cathode active material. An electrochemically active material used in the anode or negative electrode is hereinafter referred to as an anode active material. A chemical source of electrical energy or battery comprising a cathode with the cathode active material in an oxidized state and an anode with the anode active material in a reduced state is referred to as being in a charged state. Accordingly, a chemical source of electrical energy comprising a cathode with the cathode active material in a reduced state, and an anode with the anode active material in an oxidized state, is referred to as being in a discharged state.
There is a significant requirement for new types of rechargeable batteries, having high specific energy, long cycle life, safety for the user and the environment, as well as low cost. One of the most promising electrochemical systems is the lithium-sulphur system, which has high theoretical specific energy (2600 Wh/kg), safety and low cost. Sulphur or sulphur-based organic and polymeric compounds are used in lithium-sulphur batteries as a positive electrode depolarizer substance. Lithium or lithium alloys are used as depolarizer substances in the negative electrode.
Elemental sulphur (U.S. Pat. No. 5,789,108; U.S. Pat. No. 5,814,420), sulphur-based organic compounds (U.S. Pat. No. 6,090,504) or sulphur-containing polymers (U.S. Pat. No. 6,201,100, U.S. Pat. No. 6,174,621, U.S. Pat. No. 6,117,590) usually serve as a depolarizer for the positive electrode in lithium-sulphur batteries. Metallic lithium is normally used as a material for the negative electrode (U.S. Pat. No. 6,706,449). It has been suggested that it might be possible to make use of materials that can reversibly intercalate lithium for the negative electrode material. These materials include graphite (D. Aurbach, E. Zinigrad, Y. Cohen, H. Teller; “A short review of failure mechanism of lithium metal and lithiated graphite anodes in liquid electrolyte solutions”; Solid State Ionics; 2002; vol. 148; pp. 405-416), and oxides and sulphides of some metals (U.S. Pat. No. 6,319,633). However, the present applicant has not been able to find specific examples of intercalation electrodes for lithium-sulphur batteries in the available literature. It must be stressed out that it is only possible to use intercalation electrodes (negative or positive) when they are present in lithiated form. It is also necessary to take into account that intercalated compounds (where lithium is involved) are chemically active and have chemical properties close to the properties of metallic lithium.
One of the disadvantages of lithium-sulphur batteries (limiting their commercialization) is a moderate cycle life caused by a low cycling efficiency of the lithium electrode. Accordingly, twice to ten times the theoretically required amount of lithium is usually provided in lithium-sulphur batteries so as to provide a longer cycle life. In order to improve cycling of the lithium electrode, it has been proposed to add various compounds to the electrolyte (U.S. Pat. No. 5,962,171, U.S. Pat. No. 6,632,573) or to deposit protective layers of polymers (U.S. Pat. No. 5,648,187, U.S. Pat. No. 5,961,672) or non-organic compounds (U.S. Pat. No. 6,797,428, U.S. Pat. No. 6,733,924) on the electrode surface. The use of protective coatings significantly improves the cycling of the lithium electrode but still does not provide a sufficiently long cycle life for many commercial applications.
It is known that graphite intercalate electrodes possess good cycling capabilities (D. Aurbach, E. Zinigrad, Y. Cohen, H. Teller; “A short review of failure mechanism of lithium metal and lithiated graphite anodes in liquid electrolyte solutions”; Solid State Ionics; 2002; vol. 148; pp. 405-416). However, in order to use such electrodes as a negative electrode, it is necessary to have a source of lithium ions. In traditional lithium-ion batteries, this may be lithiated oxides of transition metals, cobalt, nickel, manganese and others that are depolarizers for the positive electrode.
It is theoretically possible to use the end products of sulphur electrode discharge (lithium sulphide and disulphide) as the source of lithium ions. However, lithium sulphide and disulphide are poorly soluble in aprotic electrolyte systems, and are thus electrochemically non-active. Attempts to use lithium sulphide as a depolarizer for the positive electrode in lithium-sulphur batteries have hitherto been unsuccessful (Peled E., Gorenshtein A., Segal M., Sternberg Y.; “Rechargeable lithium-sulphur battery (extended abstract)”; J. of Power Sources; 1989; vol. 26; pp. 269-271).
Lithium sulphide is capable of reacting with elemental sulphur in aprotic media so as to produce lithium polysulphides, these being compounds that have good solubility in most 0known aprotic electrolyte systems (AES) (Shin-Ichi Tobishima, Hideo Yamamoto, Minoru Matsuda, “Study on the reduction species of sulphur by alkali metals in nonaqueous solvents”, Electrochimica Acta, 1997, vol. 42, no. 6, pp. 1019-1029; Rauh R. D., Shuker F. S., Marston J. M., Brummer S. B., “Formation of lithium polysulphides in aprotic media”, J. inorg. Nucl. Chem., 1977, vol. 39, pp. 1761-1766; J. Paris, V. Plichon, “Electrochemical reduction of sulphur in dimethylacetamide”, Electrochimica Acta, 1981, vol. 26, no. 12, pp. 1823-1829; Rauh R. D., Abraham K. M., Pearson G. F., Surprenant J. K., Brummer S. B., “A lithium/dissolved sulphur battery with an organic electrolyte”, J. Electrochem. Soc., 1979, vol. 126, no. 4, pp. 523-527). The solubility of lithium polysulphides in an aprotic electrolyte system depends on the properties of the components (solvents and salts) thereof, as well as on the length of the polysulphide chain. Lithium polysulphides may undergo disproportionation in solutions according to the following schema:

Accordingly, lithium polysulphides of various lengths may be found simultaneously in the electrolyte solution at the same time, being in thermodynamic equilibrium with each other. A molecular mass distribution of the polysulphides is governed by the composition and physical/chemical properties of the electrolyte solution components. These solutions of lithium polysulphides possess high electroconductivity (Duck-Rye Chang, Suck-Hyun Lee, Sun-Wook Kim, Hee-Tak Kim “Binary electrolyte based on tetra(ethylene glycol) dimethyl ether and 1,3-dioxolane for lithium-sulphur battery”, J. of Power Sources, 2002, vol. 112, pp. 452-460) and high electrochemical activity (Taitiro Fujnaga, Tooru Kuwamoto, Satoshi Okazaki, Masashi Horo, “Electrochemical reduction of elemental sulphur in acetonitrile”, Bull. Chem. Soc. Jpn., 1980, vol. 53, pp. 2851-2855; Levillain E., Gaillard F., Leghie P., Demortier A., Lelieur J. P., “On the understanding of the reduction of sulphur (S8) in dimethylformamide (DMF)”, J. of Electroanalytical Chemistry, 1997, vol. 420, pp. 167-177; Yamin H., Penciner J., Gorenshtain A., Elam M., Peled E., “The electrochemical behavior of polysulphides in tetrahydrofuran”, J. of Power Sources, 1985, vol. 14, pp. 129-134; Yamin H., Gorenshtein A., Penciner J., Sternberg Y., Peled E., “Lithium sulphur battery. Oxidation/reduction mechanisms of polysulphides in THF solution”, J. Electrochem. Soc., 1988, vol. 135, no. 5, pp. 1045-1048).
It has been proposed to use polysulphide solutions in AES as liquid depolarizers for lithium-sulphur batteries (Rauh R. D., Abraham K. M., Pearson G. F., Surprenant J. K., Brummer S. B., “A lithium/dissolved sulphur battery with an organic electrolyte”, J. Electrochem. Soc., 1979, vol. 126, no. 4, pp. 523-527; Yamin H., Peled E., “Electrochemistry of a nonaqueous lithium/sulphur cell”, J. of Power Sources, 1983, vol. 9, pp. 281-287). Such batteries are generally known as “lithium-sulphur batteries with liquid cathodes”. The degree of sulphur utilization in such batteries with liquid sulphide cathodes depends on the nature and polarization conditions of the AES. In many cases it is close to 100% if counting full sulphur reduction and lithium sulphide formation (Rauh R. D., Abraham K. M., Pearson G. F., Surprenant J. K., Brummer S. B., “A lithium/dissolved sulphur battery with an organic electrolyte”, J. Electrochem.Soc., 1979, vol. 126, no. 4, pp. 523-527). An energy output of liquid cathodes based on lithium polysulphides is determined by their solubility. In some solvents (tetrahydrofuran, for example) sulphur solubility in the form of lithium polysulphides can reach 20M (Yamin H., Peled E., “Electrochemistry of a nonaqueous lithium/sulphur cell”, J. of Power Sources, 1983, vol. 9, pp. 281-287). The energy output of such liquid cathodes is more than 1000 Ah/l. The cycle life of lithium-sulphur batteries is also determined by the metal lithium electrode behaviour and is limited by the cycling efficiency of this electrode, which is about 80-90% in sulphide systems (Peled E., Sternberg Y., Gorenshtein A., Lavi Y., “Lithium-sulphur battery: evaluation of dioxolane-based electrolytes”, J. Electrochem. Soc., 1989, vol. 136, no. 6, pp. 1621-1625).